Apoptosis is a form of cell death which minimizes exposure of cell debris to the immune system. This programmed cell death is important in development, and in the elimination of many rapidly dividing cells. In addition, apoptosis plays a key role in eradication of self-reactive lymphocytes from the immune system. The Fas cell surface protein has been shown to mediate apoptosis when crosslinked. The lpr defect in mice is due to a mutant form of Fas, and results in systemic autoimmunity and lymphoproliferation. The phenotypically similar gld defect appears to be due to a defective Iigand for the Fas molecule. The studies proposed here will explore the connection between defective Fas-mediated apoptosis and the development of systemic autoimmune disease. We will define the extent of defective apoptosis in lpr and gld mice by identifying apoptotic cells in tissues, and will further define the phenotype of such cells by cell surface immunofluorescence staining. We will use flow cytometry to quantitate ongoing apoptosis in different autoimmune and normal cell populations, and will assess the capacity of lpr and gld cells to undergo experimental apoptosis in vitro. We will test the hypothesis that gld cells are lacking the ligand for the Fas receptor by co-culturing different gld cell populations with normal cells and assessing apoptosis. Finally, we will expand our preliminary data concerning an SLE-associated autoantibody which is directed against products released by apoptotic cells, and which is detectable using a reverse hemolytic plaque assay. We will define the specificity of this antibody, found both in murine and in human SLE, and will determine its relationship to SLE clinical manifestations.